#define _CRT_SECURE_NO_WARNINGS

#include<stdio.h>
#include<math.h>

int main()
{
	double a, b, c;
	scanf("%lf %lf %lf", &a, &b, &c);
	double p = (a + b + c) / 2;
	double s = sqrt(p * (p - a) * (p - b) * (p - c));
	printf("%.2lf\n", s);

	return 0;
}

